The present invention is directed to mechanical control cable mechanisms and, more particularly, to an apparatus for guiding a control cable that may be used to control brake or gear shifting devices on a bicycle.
Bicycle brake and shifting devices frequently use control cables of the type wherein an inner cable slides within an outer casing. Such control cables usually are connected between a controlling device, such as brake lever, a shifting lever or twist grip shifter, and a controlled device such as a brake or a transmission. Since the controlled device ordinarily is located at some distance from the controlling device, and since the controlled device ordinarily is mounted at a fixed position determined by the configuration of the bicycle, the control cable often must follow a curved path from the controlling device to the controlled device. Unfortunately, when the cable must follow a steep curve, the outer casing sometimes cannot bend satisfactorily, or else the friction between the inner cable and the outer casing caused by the curve becomes excessive. As a result, such steep curves cannot be accommodated by the control cable.
One technique used to overcome the problem of bending the outer casing to follow sharp curves involves using a roller mounted on a frame, wherein the radius of curvature of the outer peripheral surface of the roller is set to match the required curve. The frame may include a cable entry portion and a cable exit portion, wherein the inner wire enters the cable entry portion, passes around the roller, and exits the cable exit portion. While this technique is satisfactory for many applications, the radius of the roller can not be made too small because the inner cable also resists bending. For example, when an inner wire is constructed of multiple strands wound together, the optimum radius of the roller is 200 times the diameter of a single strand. For a typical strand diameter of 0.32 millimeters, the optimum roller radius then would be approximately 64 millimeters. Such a roller is extremely large and would be unacceptable for many applications. Consequently, smaller rollers are used despite the fact that such smaller rollers are not preferred. However, even though the inner cable may be forced to pass over the outer peripheral surface of such a smaller roller, the inner cable resists such bending and tends to rub against the cable entry portion and the cable exit portion of the frame. The smaller the radius of the roller, the greater the resistance to bending by the wire, and the greater the rubbing. Such rubbing can cause excessive friction, thus eliminating the benefit of using a roller to guide the inner cable around steep curves.